Metal-gate MOS transistor and method of forming the transistor with reduced gate-to-source and gate-to-drain overlap capacitance

1. A method of forming a semiconductor structure comprising: forming a gate structure that touches a semiconductor region, the gate structure having a sacrificial gate dielectric and a sacrificial gate, the sacrificial gate dielectric touching the semiconductor region, the sacrificial gate touching the sacrificial gate dielectric, the semiconductor region having a conductivity type; etching away a portion of the sacrificial gate dielectric to form a sacrificial dielectric structure and a cavity, the sacrificial dielectric structure touching the sacrificial gate and the semiconductor region, the cavity lying directly vertically below a portion of the sacrificial gate; forming a source and a drain that touch the semiconductor region after the sacrificial dielectric structure has been formed, the source and the drain each having a conductivity type that is opposite the conductivity type of the semiconductor region; forming a conformal etch resistant layer on exposed surfaces of the sacrificial gate and sacrificial dielectric structure including within the cavity; forming a non-conductive layer over the conformal etch resistant layer, the non-conductive layer filling the cavity; and forming a sidewall spacer from the conformal etch resistant layer and the non-conductive layer.

2. The method of claim 1 and further comprising removing the gate structure to form an opening after the non-conductive sidewall spacer has been formed, the opening exposing a channel region of the semiconductor region.

3. The method of claim 2 and further comprising forming a gate dielectric structure and a metal gate in the opening, the gate dielectric structure touching the semiconductor region and the conformal etch resistant layer, the metal gate touching the gate dielectric structure and the conformal etch resistant layer.

4. The method of claim 3 wherein the gate dielectric structure and the non-conductive sidewall spacer have different material compositions.

5. A method of forming a semiconductor structure comprising: forming a sacrificial gate dielectric layer over a semiconductor region; forming a sacrificial gate over the sacrificial gate dielectric layer; etching the sacrificial gate dielectric layer to undercut the sacrificial gate dielectric layer from the sacrificial gate to create a cavity, the cavity lying directly vertically below a portion of the sacrificial gate; forming a conformal etch resistant layer on exposed surfaces of the sacrificial gate and sacrificial gate dielectric layer including within the cavity; forming a non-conductive layer over the conformal etch resistant layer, the non-conductive layer filling the cavity; forming a sidewall spacer from the conformal etch resistant layer and the non-conductive layer; forming a source and a drain that touch the semiconductor region; removing the sacrificial gate and sacrificial gate dielectric layer to expose the conformal etch resistant layer; forming a gate dielectric layer on the conformal etch resistant layer; and forming a metal gate on the gate dielectric layer, such that the conformal etch resistant layer extends along the gate dielectric layer from a top region of the metal gate to the semiconductor region.

6. A method of forming a semiconductor structure comprising: forming a sacrificial gate dielectric layer over a semiconductor region; forming a polysilicon gate layer over the sacrificial gate dielectric layer; patterning the sacrificial gate dielectric layer and polysilicon gate layer to form a sacrificial gate structure including a sacrificial gate dielectric and a sacrificial gate; etching the sacrificial gate dielectric to undercut the sacrificial gate dielectric from the sacrificial gate to create a cavity, the cavity lying directly vertically below a portion of the sacrificial gate; forming a conformal nitride layer on vertical and horizontal surfaces of the sacrificial gate and on vertical surfaces of the sacrificial gate dielectric including within the cavity; forming an oxide layer over the conformal nitride layer, the oxide layer filling the cavity; forming a sidewall spacer from the conformal nitride layer and the oxide layer; forming a source and a drain that touch the semiconductor region; removing the sacrificial gate and sacrificial gate dielectric to expose the conformal nitride layer; forming a gate dielectric layer on the conformal nitride layer; and forming a metal gate on the gate dielectric layer, such that the conformal nitride layer surrounds the gate dielectric layer and the metal gate from a top region of the metal gate to the semiconductor region.